Exploring the Mystery of Iron & Nickel in Earth's Core

[danielandpenn]

I'm curious as to why there aren't heavier metals in the Earth's core than iron and nickel. Why those two? Anyone have any insight?

 

[tiny-tim]

Welcome to PF!

I'm curious as to why there aren't heavier metals in the Earth's core than iron and nickel. Why those two? Anyone have any insight?

Hi danielandpenn ! Welcome to PF!

There are … see http://en.wikipedia.org/wiki/Core_(earth_science):

The core of the Earth is still hot because it contains radioactive uranium and thorium atoms. Although the core is mostly iron, it also contains most of the Earth's uranium.

 

[Almanzo]

Nickel and iron are more abundant in the universe (or at any rate in the Solar system) than heavier substances, and might therefore dominate the core even if most of these other substances would have accumulated there.

On the other hand, some substances, like iodine, or mercury, might be mostly locked into molecules with less density than iron, or dissolved into subtances with less density than iron, like aluminium or water, and might therefore not be more abundant in the core than in the mantle or crust.

 

[Andre]

Welcome, Danie

What happens in the core is all pure hypothetical, since we have only some very indirect observations. The wiki ref of tiny-tim is certainly true qualitatively but there is no way of quantifying the concentration of heavy elements in the core as well as their contribution to the terrestrial generated heat. There are more hypotheses going around, none of which can be selected as being the right one.

The iron - nickle abundance is no doubt related to the total abundance of those elements in the solar system, whereas the heavy elements are much more rare.

Another element could be, the order in which compounds became fluid or not, during the -hypothetical- http://www.hopkins.k12.mn.us/pages/high/courses/online/astro/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm

But in the end it's all a sophisticated guess.

 

[baywax]

Welcome, Danie

What happens in the core is all pure hypothetical, since we have only some very indirect observations. The wiki ref of tiny-tim is certainly true qualitatively but there is no way of quantifying the concentration of heavy elements in the core as well as their contribution to the terrestrial generated heat. There are more hypotheses going around, none of which can be selected as being the right one.

The iron - nickle abundance is no doubt related to the total abundance of those elements in the solar system, whereas the heavy elements are much more rare.

Another element could be, the order in which compounds became fluid or not, during the -hypothetical- http://www.hopkins.k12.mn.us/pages/high/courses/online/astro/course_documents/earth_moon/earth/geologic_time/iron_catastrophe.htm

But in the end it's all a sophisticated guess.

Gee, I thought it was one big diamond at the centre... or is that Jupiter

Actually, I'd like to ask a question related to this one... and that is why is gold associated with volcanic activity? Does it get separated out by extreme heat or is it of greater abundance in the liquid magma surfacing during a volcanic eruption?

Many thanks!

 

[mgb_phys]

... and that is why is gold associated with volcanic activity?

Gold isn't very chemically active but is soluable at very high pressures. It is carried along cracks by steam/fluids from volcanoes and is deposited out as they cool. So are lots of other metals, there a bunch of elements like platinum and telurium that you find along with gold.

 

[baywax]

Gold isn't very chemically active but is soluable at very high pressures. It is carried along cracks by steam/fluids from volcanoes and is deposited out as they cool. So are lots of other metals, there a bunch of elements like platinum and telurium that you find along with gold.

Ah, right! The chinese who built most of the CN and CPR railways across Canada would keep what they called "white gold" (platinum) in their old opium jars. It was considered useless at the time by the CDN and USA placer miners, but the "Coolies" kept it just the same.

 

[Almanzo]

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core. One would have to conclude that elements heavier than carbon (such as iron and nickel) are relatively rare on Jupiter. In that case iron and nickel cannot be all that common in the Solar System, and one would have to wonder why they are common on Earth.

That is quite different from the case for hydrogen. Hydrogen is the most common element in the universe, but Earth cannot keep hold of it (except in compounds with heavier elements, such as oxygen), because it is light enough to escape from Earth's gravity well. Jupiter is more massive, and was therefore able to retain more hydrogen.

Carbon, however, would not escape from Earth, at least not over the present age of the Solar System. If a large part of Jupiter is made of carbon, that would be strange.

 

[baywax]

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core. One would have to conclude that elements heavier than carbon (such as iron and nickel) are relatively rare on Jupiter. In that case iron and nickel cannot be all that common in the Solar System, and one would have to wonder why they are common on Earth.

That is quite different from the case for hydrogen. Hydrogen is the most common element in the universe, but Earth cannot keep hold of it (except in compounds with heavier elements, such as oxygen), because it is light enough to escape from Earth's gravity well. Jupiter is more massive, and was therefore able to retain more hydrogen.

Carbon, however, would not escape from Earth, at least not over the present age of the Solar System. If a large part of Jupiter is made of carbon, that would be strange.

Very interesting. Do we know what the predominant metal/other is at Mar's core? Does the volcanic activity of Io, next to Jupiter, indicate a core content similar to earth's?

 

[mgb_phys]

Do we know what the predominant metal/other is at Mar's core?

We don't have direct seismic data for Mars but it is thought to be similair to Earth. The difference is that Mars is smaller and so it's core cooled and solidifed and so has no (significant) magnetic field.

Does the volcanic activity of Io, next to Jupiter, indicate a core content similar to earth's?

The volcanic activity of Io is due to the energy input from Jupiter, from tidal forces, it is too small to have a liquid core purely from internal heat sources. It does look like it has an iron core from magnetic field measurements made by the Galileo probe.

 

[Gokul43201]

Gee, I thought it was one big diamond at the centre... or is that Jupiter

If the core of Jupiter is made of diamond, that would mean that carbon is dominating Jupiter's core.

To my knowledge, there is no real evidence of any significant amounts of carbon in Jupiter's core. The diamond core theory was made up by Arthur C. Clarke for one of his later Odyssey novels.

There is definitely more realistic speculation of metallic hydrogen in jupiter's core, but I don't think that's been verified either.

 

[sketchtrack]

To my knowledge, there is no real evidence of any significant amounts of carbon in Jupiter's core. The diamond core theory was made up by Arthur C. Clarke for one of his later Odyssey novels.

There is definitely more realistic speculation of metallic hydrogen in jupiter's core, but I don't think that's been verified either.

Wouldn't it be too hot for diamonds in the core of a planet? Wouldn't they burn up, or would the pressure prevent that?

 

[baywax]

I'm sorry, I mentioned the diamond centre of Jupiter as a joke... sort of sarcastic kind of thing.

 

[mgb_phys]

Wouldn't it be too hot for diamonds in the core of a planet?

You need oxygen for things to burn, also Carbon has a rather high boiling point (4000K)
Diamond isn't the most stable form of Carbon, diamonds (slowly) decay into graphite at room temperature - I don't know about at the pressure in the centre of a planet

 

[baywax]

You need oxygen for things to burn and Carbon has a rather high boiling point (4000K)
Diamond isn't the most stable form of Carbon, diamonds (slowly) decay into graphite at room temperature - I don't know about at the pressure in the centre of a planet

So, how long till the engagement diamond turns into a pencil? Probably longer than the license takes to decompose, eh?.

 

[Andre]

Boiling point of carbon is even a bit higher, 5100K and the melting point (grafite) is 3773K. The temperature of the Earth core is estimated at about http://mediatheek.thinkquest.nl/~ll125/en/core.htm . But the tremendous pressure would probably prevent carbon to be fluid.

 

[sketchtrack]

Could one assume that if there is carbon in or near the core, it is probably in diamond form?

 

[mgb_phys]

Could one assume that if there is carbon in or near the core, it is probably in diamond form?

It's probably dissolved in molten iron and so doesn't have any crystal structure.

 

[Almanzo]

Suppose that the core of some planet is nearly pure carbon. Then the carbon would not burn or react with other substances. Whether it would be diamond, graphite or something else, not known to us in dayly life, would depend on carbon's diagram of state. This is a parameter space with pressure and temperature as its two parameters. It is divided into several regions (which may overlap) where several forms of carbon may exist (be stable against change into another form). If the entire diagram were known, one might find the point corresponding to the pressure and temperature in the core, and see whether diamond can exist there. (That is, if the pressure and temperature are known. But maybe the diamond region extends to quasi-infinite pressures and temperatures? Quasi-infinite, not really infinite, because eventually degenerate matter would form.)

The question is, whether the state diagram for carbon is completely known. (It might be completely known from first principles, not from experiment, of course.) And the same question might be asked about nickel and iron. The material in the Earth's core might be unusual, too.

 

[danielandpenn]

Are you saying that the nickel and iron in the core may be different from the nickel and iron as we know it because of the intense pressure and temps?

 

[Almanzo]

Yes. I do not know whether they are different, but they might be as different from the usual iron and nickel as diamond is from graphite. (Chemically, they would of course still be the same materials. Only the crystal structure would be different.)

 

[Astronuc]

Are you saying that the nickel and iron in the core may be different from the nickel and iron as we know it because of the intense pressure and temps?

Elementally the iron and nickel would not be different, but given a pressure of about 350 GPa, the distances between nuclei, or lattice parameters of any crystal arrangement would be very small. But the core is considered to be liquid.

http://www.soest.hawaii.edu/GG/ASK/Earth's_core.html
http://pubs.usgs.gov/gip/interior/
http://www.geol.umd.edu/~mcdonoug/Encyclopedia_Geomagnetism_Paleomagnetism.pdf
http://mahi.ucsd.edu/cathy/SEDI2002/ABST/SEDI1-2.html

As others have indicated, the heaviest elements are quite rare, and most metals are in the form of oxides, metallic oxides, e.g. silicates and aluminates, carbonates or other complex metal oxides, and sulfides.

List of Periodic Table Elements Sorted by Abundance in Earth's crust
http://www.science.co.il/Ptelements.asp?s=Earth

http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements

 

[danielandpenn]

Okay...thank you very much!

 

[gdp]

There is another point that has not yet been raised except in passing, which is that in nature, because the various elements have various chemical affinities for each other, most elements in nature do not occur in "pure" form, but rather in the form of chemical compounds or mixtures.

The Earth's core is believed to be mostly composed of a mixture of http://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements" In addition to iron, cobalt, and nickel, the siderophiles include manganese, molybdenum, gold, and the platinum and palladium group elements.

Secondarily, in the core one also expects to find enhanced abundances of the http://en.wikipedia.org/wiki/Goldschmidt_classification#Chalcophile_elements" --- most notably sulfur itself, which is believed by some to be a significant constituent of the liquid "outer core." (Mercury, BTW, happens to be a chalcophile.)

By contrast, uranium and most uranium compounds are http://en.wikipedia.org/wiki/Goldschmidt_classification#Lithophile_elements" and are therefore believed to have become largely concentrated mostly in the Earth's lithosphere (i.,e., its "crust"), rather than in its mantle or core --- despite the relatively high specific gravities of uranium and most of its compounds. [Note that chemical bonds are basically `electromagnetic' in nature, and that gravity is actually an extremely weak force compared to the electromagnetic force. (Relatively speaking, the electromagnetic force is about ~10⁴⁰ times stronger than the gravitational force.) Hence, the chemical bonds between uranium and the other lithophilic elements are easily capable of preventing uranium from sinking to the core under the influence of the Earth's relatively weak gravitational field.]

There is a heterodox hypothesis by maverick nuclear chemist http://en.wikipedia.org/wiki/J._Marvin_Herndon" that the core is primarily composed of nickel silicide rather than metallic nickel-iron alloy. Nickel silicide has enough of an affinity for uranium that, despite uranium's relatively low cosmic abundance, there might have been enough uranium dissolved in the primitive core for it to later separate out to form a small innermost "kernel" of uranium sulfide with a diameter of ~1--10 km at the center of the inner core. Herndon also believes that this inner "kernel" functions as an intermittent fast fission reactor that drives various geological processes. However, very few "mainstream" geologists take Herndon's "nuclear planet" hypothesis seriously, and there is currently very little evidence of even a circumstantial nature to support it.

 

[baywax]

There is another point that has not yet been raised except in passing, which is that in nature, because the various elements have various chemical affinities for each other, most elements in nature do not occur in "pure" form, but rather in the form of chemical compounds or mixtures.

The Earth's core is believed to be mostly composed of a mixture of http://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements" In addition to iron, cobalt, and nickel, the siderophiles include manganese, molybdenum, gold, and the platinum and palladium group elements.

Secondarily, in the core one also expects to find enhanced abundances of the http://en.wikipedia.org/wiki/Goldschmidt_classification#Chalcophile_elements" --- most notably sulfur itself, which is believed by some to be a significant constituent of the liquid "outer core." (Mercury, BTW, happens to be a chalcophile.)

By contrast, uranium and most uranium compounds are http://en.wikipedia.org/wiki/Goldschmidt_classification#Lithophile_elements" and are therefore believed to have become largely concentrated mostly in the Earth's lithosphere (i.,e., its "crust"), rather than in its mantle or core --- despite the relatively high specific gravities of uranium and most of its compounds. [Note that chemical bonds are basically `electromagnetic' in nature, and that gravity is actually an extremely weak force compared to the electromagnetic force. (Relatively speaking, the electromagnetic force is about ~10⁴⁰ times stronger than the gravitational force.) Hence, the chemical bonds between uranium and the other lithophilic elements are easily capable of preventing uranium from sinking to the core under the influence of the Earth's relatively weak gravitational field.]

There is a heterodox hypothesis by maverick nuclear chemist http://en.wikipedia.org/wiki/J._Marvin_Herndon" that the core is primarily composed of nickel silicide rather than metallic nickel-iron alloy. Nickel silicide has enough of an affinity for uranium that, despite uranium's relatively low cosmic abundance, there might have been enough uranium dissolved in the primitive core for it to later separate out to form a small innermost "kernel" of uranium or uranium sulfide with a diameter of ~1--10 km at the center of the inner core. Herndon also believes that this inner "kernel" functions as an intermittent fast fission reactor that drives various geological processes. However, very few "mainstream" geologists take Herndon's "nuclear planet" hypothesis seriously, and there is currently very little evidence of even a circumstantial nature to support it.

Nice work GDP...

 

[danielandpenn]

Wow! How do you know all this GDP? I can really grasp that! Thanks.

 

[DrClapeyron]

By which process is uranium created? Which process is iron created? Simply, iron production was favored during the formation of our early solar nebula. Chalk it up to thermodynamics. Better question: why is the Earth's core iron and its mantle silicate?

 

[gdp]

Wow! How do you know all this GDP? I can really grasp that! Thanks.

Well, I'm an old man, and I've had a lot of time to read...

But seriously --- I have a long-standing interest in how the geomagnetic dynamo works, which means I've picked up a fair amount of knowledge over the last three decades or so about the structure and formation of the core and the rest of the Earth in the process. (Plus, my memory is still reasonably intact... )

 

[gdp]

By which process is uranium created? Which process is iron created?

The current model is http://en.wikipedia.org/wiki/R-process" within the core of said supernova's precursor, respectively.

Simply, iron production was favored during the formation of our early solar nebula. Chalk it up to thermodynamics.

Simply, no. Nucleosynthesis occurred prior to the formation of the solar nebula, and not during its formation.

Moreover, "simple thermodynamics" alone is by no means sufficient to explain http://en.wikipedia.org/wiki/Stellar_nucleosynthesis" --- the formation of the heavier elements by fusion reactions within the cores of massive stars. One must also take into account quantum physics, nuclear structure, and nuclear reactions.

Thermodynamics is quite relevant to understanding the radial variation of elemental abundances within the solar nebula: The higher temperatures within the inner disk led to a selective depletion of the more volatile fractions of the primordial nebular material from the inner disk, and a corresponding transfer to and enhancement of volatiles in the portions of the disk outside the so-called http://en.wikipedia.org/wiki/Frost_line_(astrophysics)" However, these elements were already preexisting within the primordial nebula --- they were not `produced' by it.

Better question: why is the Earth's core iron and its mantle silicate?

Quite simply, because they are mutually immiscible substances, and the iron fraction is denser than the silicate fraction. The two fractions separated in the primordial molten Earth for exactly the same reason that oil and vinegar separate in a bottle of salad dressing.

 

[Astronuc]

Quite simply, because they are mutually immiscible substances, and the iron fraction is denser than the silicate fraction. The two fractions separated in the primordial molten Earth for exactly the same reason that oil and vinegar separate in a bottle of salad dressing.

Sounds much like the separation of the slag on top of liquid iron in the iron and steel making process.

http://www.tfhrc.gov/hnr20/recycle/waste/ssa1.htm

 

[gdp]

Sounds much like the separation of the slag on top of liquid iron in the iron and steel making process.

http://www.tfhrc.gov/hnr20/recycle/waste/ssa1.htm

Yes, exactly. There is no fundamental difference in the physical principles involved.

Indeed, one can view the mantle as the "slag" that "floated to the top" when the core material settled out, and the lithosphere or crust as the "slag" that likewise "floated to the top" of the mantle. In both cases, one had two mutually immiscible substances that separated due to differences in their specific gravities.